The present invention relates to a multi-stage detection system in a digital demodulator for demodulating a received signal by differential detection.
Digital demodulators are widely used in the telecommunication field. A digital modulator which demodulates a received signal by differential detection can be efficiently used in a mobile communication system. The signal to be received by the digital demodulator is transmitted as a transmitted signal from a transmitter. For example, this transmitted signal is produced by differential phase shift keying in the transmitter. That is, the transmitted signal has phase information.
The coherent detection methods obtain the phase information by reproducing the carrier frequency and phase correctly (see, for example, Y. Matsumoto, S. Kubota and S. Kato, "A New Burst Coherent Demodulator for Microcellular TDMA/TDD Systems", IEICE Trans. Commun. Vol. E77-B, No. 7, pp. 927-933, July, 1994). If the carrier frequency and phase can be reproduced correctly, the demodulation characteristics are superior to that of the other kind of demodulators. However, it is complicated to realize the carrier synchronization and since the modulated signal is always transmitted under a fading environment in mobile communication, it is also quite difficult to reproduce the carrier frequency and phase exactly.
The conventional differential detection methods calculate the phase difference between the two adjacent symbols of the received signal (see, for example, C. Liu and K. Feher, "Proposed .pi./4-QPSK with Increased Capacity in Digital Cellular Systems", ICC'92, pp. 316-318, 1992, and H. Tomita, Y. Yokoyama, T. Matsuki, "DIGITAL INTERMEDIATE FREQUENCY DEMODULATION TECHNIQUE FOR CELLULAR COMMUNICATION SYSTEMS", IEEE GLOBECOM'90, San Diego, Calif., pp. 1827-1831, December 1990), and a decision rule is used to obtain the transmitted information. Although the differential detection methods are effective under the fading environment, the static characteristics are inferior to those of the coherent detector.
The multiple-symbol method uses more than two symbols for improving the static characteristics of the above differential detectors (see, for example, D. Divsalar and M. Simon, "Multiple-Symbol Differential Detection of MPSK", IEEE Trans. Commun. Vol. 38, No. 3, pp. 300-308, March 1990). However, it becomes very complicated to realize the multiple-symbols detector.